SpamBayes is a tool used to segregate unwanted mail (spam) from the mail
you want (ham). Before SpamBayes can be your spam filter of choice you
need to train it on representative samples of email you receive. After
it's been trained, you use SpamBayes to classify new mail according to
its spamminess and hamminess qualities.

When SpamBayes filters your email, it compares each unclassified message
against the information it saved from training and makes a decision
about whether it thinks the message qualifies as ham or spam, or if it's
unsure about how to classify the message. It then passes this
information on to your mail client. Unless you are using IMAP or
Outlook, this means it adds a header to each message,
X-SpamBayes-Classification: spam|ham|unsure. You can then filter on this
header, to file away suspected spam into its own mail folder for
example. IMAP and Outlook both have the capacity to do the filtering
themselves, so the header is not necessary.
